The layer of a cell membrane surface consisting of sugar chains and proteins is a site for the cell to express a response to the external environment, such as intercellular recognition. Since molecular recognition and the conversion of information are performed at this site, it is thought that modifying the property of cell membrane surfaces enables diversification of the cell recognition. For example, studies have been conducted to non-individualize cells by modifying the cell membrane surfaces with synthetic polymers.
Japanese Patent Unexamined Publication (Kohyo) No. 2000-507849 discloses a method for covalently modifying cell surfaces by allowing biologically compatible polymers to react with functional groups present on the cell surfaces. Another method for modifying cell surfaces with GPI-anchored protein is reported in The FASEB Journal, 10, 574–586 (1996). Moreover, Polymer Preprints, Japan, 47, 10, 2499–2500 (1998) and Protein/Nucleic Acid/Enzyme (Tanpaku-shitsu, Kakusan, Kohso), 45, 11, 1859–1864 (2000) disclose a method for modifying cell surfaces, which comprises the step of allowing the reactive groups of the polymer ends of polyacrylamides, used as water-soluble polymers, to react with membrane proteins or sugar chains; a method for modifying cell surfaces through binding of ligands to receptors on the cell membranes; and a method for modifying cell surfaces using hydrophobic anchors.
As described above, several methods are known to modify cell surfaces by binding reactive groups of polymer ends to cell membrane proteins and sugar chains. However, such a direct chemical modification on cell membranes may alter properties of substances on the cell surfaces or may damage the cells. Further, the method using GPI-anchored protein has problems in that GPI-anchored proteins are hardly obtainable, and therefore its applicability is limited. Furthermore, by the method using hydrophobic anchors, cell surfaces are modified only with polyacrylamide chains. Although the method successfully achieves suppression of cell aggregation, stability after modification becomes poor, and it has not been verified that polyacrylamide chains can be immobilized on the cell membrane surfaces. Moreover with this method, it is impossible to bind physiologically active substances to cell surfaces.
The Journal of Immunology, 2433–2443 (2000) discloses a method which comprises the steps of anchoring chelate compounds containing hydrocarbon groups to cell membranes, and then allowing physiologically active substances to ionically bind to the chelate compounds. However, the method has problems in that since physiologically active substances are ionically bound to cell membranes, the binding is susceptible to changes of salt concentration or pH and is consequently unstable, and its applicability to cells is also limited. In addition, since the chelate compounds is highly lipophilic as they contain no hydrophilic group and the compounds can hardly be uptaken by cells, a means including preparation of liposomes and their fusion with cells has been used for intracellular incorporation of the compounds. However, the modification of cell membranes by this means is not convenient.
As explained above, no method has been known to date by which properties of cell membranes are modified by covalently binding physiologically active substances or the like to cell membranes without causing any damage to the cells.